Oof i have one that is both best and worst.
It was for my 7th or 8th birthday. My family moved back to my hometown, but i never grew up there, so i did not know anyone my age. I got into new school and started second grade of elementary school. Everyone already knew each other from first grade so me being very shy, I had a hard time making friends. My birthday comes, and I decided i wanted to have my birthday party in McDonald, since it was very hyped at that time (and very expensive, since this was small town in eastern europe). Everyone from my school class was invited, like 30 kids and some of my cousins as well, all similar age to me, but when the time came, almost no one from school was there. Besides my cousins and family only two boys from school came.
I dont remember much, but i do remember having great time even though there was only 7 kids, while McDonald expected almost 40. I also remember being disappointed that no one else came, but i knew it was because no one else really knew me well, and because i did not put any effort in getting to know everyone in class at school. I imagine that McDonald employees were feeling bad for me and for the whole awkward situation with only few kids being there while it was payed for full party.
It was worst one, because i never made a birthday party again, and i never wanted to feel such disappointment again. From then on, I always hated my birthday, and it was celebrated at home with my extended family and few best friends that I had, just stopping by for some cake. During high school and university, I usually just payed drinks for couple of friends during the regular night out around birthday, it was never planned.
It was best one, because i never had one again, but also because I realized which kids from school liked me enough to hangout with me. One of the kids that showed up back then, is still my best friend after 20 years, and we still do everything together regardless if it is just hanging out or making big life decision. Other kid was also my best friend for 10 years, but we lost touch during high school and university because we went to different schools. I am still in contact with him, but life went on so we just catch up once a year and that's it.